DESCRIPTION (as provided by investigator): The goal of this proposal is to study the contribution of the thymus to the pathogenesis of SIV infection in rhesus macaques. The first approach to be used is primarily based upon our recently developed technique to quantify the number of recent thymic emigrants (RTE) in blood by measuring a major excisional DNA byproduct (termed alpha1 circle) of T-cell receptor (TCR) rearrangement. By monitoring the decay of alpha1 circle concentration in the peripheral blood following surgical removal of the thymus (thymectomy), the intrinsic thymic output can be calculated. The second approach will be to in situ label thymocytes by injection of the thymus with a fluorescent dye CFSE, and to follow the fluorescent-labeled RTE in the peripheral blood. These two different approaches will provide a genetic and cellular marker for the RTE, so that the intrinsic thymic output can be accurately estimated. The impact of SIV infection on the thymic output will also be studied by comparing the alpha1 circle concentration before and after SIV infection. In addition, thymic contribution to peripheral T lymphocyte dynamics will be analyzed by sequential measurement of the alpha1 circle concentration in various cell populations, in particular in the naive and memory cells. The regenerative capacity of the thymus will be further studied by measuring the changes of the alpha1 circle in the blood after infected animals are treated with the antiretroviral drug PMPA. The source and mechanism of the increase in T cells following antiretroviral treatment will then be addressed by comparing the surface phenotypes of lymphocytes and alpha1 circle numbers before and after treatment. The information from the proposed studies will contribute to our understanding of HIV-1 pathogenesis, in particular the role of the thymus in the regeneration of T lymphocytes during the course of infection and after treatment with highly active antiretroviral therapy (HAART).